Air bags for protection of motor vehicle occupants must be inflated by the gas-generating composition within a fraction of a second, and they are generally constructed so that their gas content is released at a controlled rate. The propellant formed for such air bags must not contain any toxic components.
Alkali and alkaline earth metal azides in particular, which form nonpoisonous gas consisting essentially of nitrogen when reacted with an inorganic oxidizing agent, come into consideration as the gas supplying component of such compositions. Alkali and alkaline earth metal oxides, which form during oxidation, are relatively difficult to separate and may reach the interior of the vehicle. To make the oxide harmless, it is known, for example, from German Aulegeschrift No. 2,236,175, that silicon dioxide may be added to the gas-generating composition. The silicon dioxide and the alkali and alkaline earth metal oxides form a glassy slag, the separation of which presents no problems.
The composition disclosed by German Auslegeschrift No. 2,236,175, as used in practice, contains 56% sodium azide on a weight basis, a relatively high proportion. Moreover, sodium azide is highly toxic, comparable in this respect to potassium cyanide. Due to the constant increase in the number of motor vehicles which are equipped with such protection equipment, the disposal problems which arise when scrapping are appreciable. These problems result both from direct contamination of the environment, particularly soil and subterranean water with this highly toxic salt, and from the reaction of sodium azide on the scrap heap with acids. For example, sodium azide can come into contact with bacterial acids to form highly explosive heavy metal azides.
Therefore, every effort is made to reduce the azide content of such compositions or to make do without azides. For example, azide-free compositions based on solid rocket fuels have been disclosed in German Auslegeschriften Nos. 2,334,063 and 2,222,506. However, these compositions have a serious disadvantage; carbon monoxide and other toxic gases are formed from the carbon containing components thereof.
To avoid carbon monoxide formation, the use of oxygen-free oxidizers, such as chromium chloride, molybdenum disulfide or iron fluoride and tetrazoles as a nitrogen source has been disclosed in European Patent No. 0,055,904. In these reactions, a propellant is formed which contains free metal, i.e. chromium, molybdenum or iron, and in some cases, substances which are even substantially more toxic, such as potassium cyanide. Furthermore, in view of the long time span over which an air bag must be usable, for example, more than ten years, the chemical stability of this composition leaves much to be desired.
According to German Offenlegungsschrift No. 2,407,659, a composition containing silicon nitride, sodium azide and ammonium perchlorate generates gas by reacting according to the following equation:
Si.sub.3 N.sub.4 +3NaN.sub.3 +3NH.sub.4 ClO.sub.4 .fwdarw.3SiO.sub.2 +3NaCl+8N.sub.2 +6H.sub.2 O (1)
As shown in equation (1), the reaction of this composition results in the formation of a relatively large amount of water, which condenses in the air bag. Further, due to the resulting heat of condensation, the air bag is heated, and aside from the mechanical stress, the bag material also has to withstand appreciable thermal stress. This problem is difficult to solve, because it runs counter to the desire, or necessity, of making the bag material as thin as possible, due to space constraints.
Another disadvantage of the composition disclosed by German Offenlegungsschrift No. 2,407,659 is that the ammonium perchlorate, which is used together with sodium azide, becomes acidic in contact with water and reacts with the formation of highly explosive and highly toxic hydrazoic acid. In practice, the complete absence of water cannot be attained, or requires an inordinate effort and expense.
Even though it is not evident from equation (1), in practice, the formation of hydrochloric acid, a toxic substance, from ammonium perchlorate is unavoidable. Finally, as shown in equation (1), sodium chloride is produced, and moreover, it is produced in finely divided form. The removal of sodium chloride creates difficulties; if it is not removed, it exerts an irritant effect on the occupants of the motor vehicle.